1. Field of the Invention
The present invention relates to a magnetic field detecting element and a manufacturing method thereof, and more particularly, to the structure of a magnetic field detecting element having a plurality of free layers.
2. Description of the Related Art
As a reproduction element of a thin film magnetic head, GMR (Giant Magneto Resistance) elements are known. Hitherto, CIP (Current In Plane)-GMR element, in which sense current flows in a direction that is horizontal to the film surface of the element, have been mainly used. In recent years, however, in order to cope with higher density recording, elements have been developed in which sense current flows in a direction that is perpendicular to the film surface of the element. TMR elements utilizing the TMR (Tunnel Magneto-Resistance) effect, and CPP (Current Perpendicular to the Plane) elements utilizing the GMR effect are known as the elements of this type. In particular, the CPP element has high potential because it has low resistance as compared to the TMR element and because it exhibits high output even with a narrow track width as compared to the CIP element.
The CPP element includes a stack having a magnetic layer (free layer) whose magnetization direction changes in accordance with an external magnetic field, a magnetic layer (pinned layer) whose magnetization direction is fixed with respect to the external magnetic field, and a non-magnetic intermediate layer sandwiched between the pinned layer and the free layer. This stack is also called a spin-valve film. On both sides of the spin-valve film with regard to the track width direction, bias magnetic layers for applying a bias magnetic field to the free layer are provided. The free layer is magnetized into a single magnetic state by a bias magnetic field emitted from the bias magnetic layers. This provides an improvement in linearity of a change in resistance with respect to a change in an external magnetic field and an effective reduction in Barkhausen noise. A relative angle between the magnetization direction of the free layer and the magnetization direction of the pinned layer changes in accordance with an external magnetic field, and as a result, electric resistance of sense current that flows in a direction perpendicular to the film surface of the spin-valve film is changed. By making use of this property, external magnetization is detected. The spin-valve film is magnetically shielded by shield layers on both sides thereof with regard to the direction of stacking. The direction of stacking of the spin-valve film corresponds to the circumferential direction of a recording medium when a thin film magnetic head is incorporated into a hard disc drive. Therefore, the shield layers have a role of shielding a magnetic field emitted from adjacent bits on the same track of the recording medium.
In recent years, higher track recording density is desired. However, an improvement in track recording density requires reduction in spacing between upper and lower shield layers (a gap between shields). In order to achieve this, a decrease in thickness of the spin-valve film is required. However, there is large limitation that originates from the layer configuration in the conventional CPP elements. Specifically, since the pinned layer requires that the magnetization direction be firmly fixed without being influenced by an external magnetic field, a so-called synthetic pinned layer is usually used. The synthetic pinned layer includes an outer pinned layer, an inner pinned layer, and a non-magnetic intermediate layer which consists of Ru or Rh and which is sandwiched between the outer pinned layer and the inner pinned layer. Moreover, an antiferromagnetic layer is provided in contact with the outer pinned layer in order to fix the magnetization direction of the outer pinned layer. The antiferromagnetic layer typically consists of IrMn. In the synthetic pinned layer, the antiferromagnetic layer is coupled to the outer pinned layer via exchange-coupling so that the magnetization direction of the outer pinned layer is fixed. The inner pinned layer is antiferromagnetically coupled to the outer pinned layer via the non-magnetic intermediate layer so that the magnetization direction of the inner pinned layer is fixed. Since the magnetization directions of the inner pinned layer and the outer pinned layer are anti-parallel to each other, magnetization of the pinned layer is limited as a whole. Despite such a merit of the synthetic pinned layer, however, a large number of layers are required to constitute a CPP element that includes the synthetic pinned layer. This imposes limitation on a reduction in the thickness of the spin-valve film.
Meanwhile, a novel layer configuration that is entirely different from that of the above-mentioned conventional spin-valve film has been proposed in recent years. In “Current-in-Plane GMR Trilayer Head Design for Hard-Disk Drives” (IEEE TRANSACTIONS ON MAGNETICS, Vol. 43, No. 2, February 2007), a stack used for the CIP element, which includes two free layers and a non-magnetic intermediate layer that is sandwiched between the free layers, is disclosed. Each of the magnetization direction of the free layers changes in accordance with an external magnetic field. A bias magnetic layer is provided on the side of the stack opposite to the air bearing surface, and a bias magnetic field is applied in a direction that is perpendicular to the air bearing surface. The magnetization directions of the two free layers adopt a certain relative angle because of the magnetic field applied from the bias magnetic layer. If an external magnetic field is applied in this state, then the magnetization directions of two free layers are changed. As a result, the relative angle between the magnetization directions of the two free layers is changed, and accordingly, electric resistance of sense current is changed. By making use of such a property, it becomes possible to detect an external magnetic field. Moreover, in U.S. Pat. No. 7,035,062, an example is disclosed in which such a layer configuration is applied to the CPP element. Such a layer configuration using two free layers has a potential for facilitating a reduction in the gap between the shield layers, because it does not require the conventional synthetic pinned layer and the antiferromagnetic layer, allowing a simplified layer configuration.
However, such a stack using two free layers has the problem described below. First, when the stack thickness is decreased, the thickness of the bias magnetic layer is decreased together depending on the decrease in the stack thickness. Next, since the bias magnetic layer is provided facing only one surface of the stack, which is different from the conventional art, the magnetic field itself is apt to be dispersed, and efficient application of a magnetic field to the free layer is difficult. For these reasons, it is difficult to ensure a magnetic field intensity of the bias magnetic layer that is necessary for magnetizing the free layers into a single magnetic domain. To solve this problem, it is necessary to ensure the thickness of the bias magnetic layer. However, if the stack thickness is determined in a manner that it corresponds to the thickness of the bias magnetic layer, a large decrease in the gap between the shields cannot be expected.